This invention relates to expansible duct seal constructions for mixers of the type wherein various mixing and reacting is occurring, for example, mixers in which reactants produce fumes which cannot be allowed to escape into the atmosphere of the plant where mixing is occurring. One of the problems encountered with conventional large batch mixers and processors which, for instance, are using processing temperatures which may reach 700.degree. F., is gas leakage due to the fact of differential thermal expansion of the mixer vessel and its cover. Presently, covers for these mixers have been bolted around their rectangular perimeter to the lateral flange provided on the upper end of the mixer housing, and the openings in the cover for the bolts have been enlarged to permit relative movement. While the obvious solution to the problem is to circulate a temperature changing, heating medium to the cover, it has been determined that this is not a viable solution to the problem, because, when the batch of reactants are loaded to the mixers, frothing and splashing causes some material to stick to the cover. If the cover were as hot as the vessel, this material would react and finally char, and then drop off into subsequent batches causing irreversible combination.
Another approach to solving the problem involves running the reaction process at slightly over atmospheric pressure to reduce the volume of fumes which escape. This may result in an altered product when the process should be run at a pressure slightly below atmospheric pressure, and, of course, the escape of fuming acid and other products is to be avoided, since it seriously deteriorates the working environment.